Conventionally, a ground electrode is attached to an end surface of a metallic shell by the following method: the metallic shell is fixedly held between mutually facing chucks so as to dispose the ground electrode at a predetermined position, after which the ground electrode is welded to the metallic shell. Resistance welding is performed for this welding process (See WO2010/053099). In this process of resistance welding, in order to reduce contact resistance between the chucks and the metallic shell, a substantially circular columnar portion of the metallic shell is held between the chucks whose cavities collectively assume a cylindrical shape having a diameter substantially equal to that of the circular columnar portion of the metallic shell (See Japanese Patent Application Laid-Open (kokai) No. 2009-16129).
According to the above technique, in the theoretical case where the diameter of the collectively cylindrical cavities of the chucks is equal to the outside diameter of the substantially circular columnar portion of the metallic shell or in the case where the difference in diameter between them is to such an extent as to be absorbable through elastic deformations of the metallic shell and the chucks, the collectively cylindrical cavities of the chucks and the substantially circular columnar portion of the metallic shell come into surface contact with each other. However, in other cases, the following states of contact arise. Specifically, in the case where the diameter of the collectively cylindrical cavities of the chucks is smaller than the outside diameter of the substantially circular columnar portion of the metallic shell, the metallic shell fails to be sufficiently received in the cavities of the chucks and comes into contact with only opposite ends of the recess of a chuck. In the case where the diameter of the collectively cylindrical cavities of the chucks is larger than the outside diameter of the substantially circular columnar portion of the metallic shell, the metallic shell comes into contact with the deepest region of the recess of a chuck.
That is, according to the above technique, the contact area between the metallic shell and the chucks may possibly vary greatly depending on the outside diameter of the substantially circular columnar portion of the metallic shell being substantially equal to, greater than, or smaller than the diameter of the collectively cylindrical cavities of the chucks. As a result, current which flows in resistance welding varies greatly due to attachment errors and dimensional errors produced in the course of manufacture of the metallic shell and the chucks. Therefore, the quality of attachment of the ground electrode to the metallic shell becomes unlikely to be secured.
Also, in the case where the attachment positions of the facing chucks are misaligned from each other, the metallic shell and the chucks fail to come into theoretical surface contact with each other; as a result, resistance welding cannot be performed as expected.